In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) is heretofore used. Specifically, the PS plate is exposed through a mask, for example, a lith film, and then subjected to development processing, for example, with an alkaline developer to remove the unnecessary image-recording layer corresponding to the non-image area by dissolving while leaving the image-recording layer corresponding to the image area, thereby obtaining the lithographic printing plate.
Due to the recent progress in the technical field, nowadays the lithographic printing plate can be obtained by a CTP (computer-to-plate) technology. Specifically, a lithographic printing plate precursor is directly subjected to scanning exposure using laser or laser diode without using a lith film and developed to obtain a lithographic printing plate.
With the progress described above, the issue on the lithographic printing plate precursor has transferred to improvements, for example, in image-forming property corresponding to the CTP technology, printing property or physical property. Also, with the increasing concern about global environment, as another issue on the lithographic printing plate precursor, an environmental problem on waste liquid discharged accompanying the wet treatment, for example, development processing comes to the front.
In response to the environmental problem, simplification of development or plate making or non-processing has been pursued. As one method of simple plate making, a method referred to as an “on-press development” is practiced. Specifically, according to the method after exposure of a lithographic printing plate precursor, the lithographic printing plate precursor is mounted as it is on a printing machine without conducting conventional development and removal of the unnecessary area of image-recording layer is performed at an early stage of printing process.
In the plate making operation as described above, a system using a lithographic printing plate precursor capable of being handled in a bright room or under a yellow lamp and a light source is preferred from the standpoint of workability. Thus, as the light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 or a solid laser, for example, YAG laser, is used.
As the lithographic printing plate precursor capable of undergoing on-press development, for instance, a lithographic printing plate precursor having provided on a hydrophilic support, an image-recording layer (heat-sensitive layer) containing microcapsules having a polymerizable compound encapsulated therein is described in Patent Document 1 or 2. A lithographic printing plate precursor having provided on a support, an image-recording layer (photosensitive layer) containing an infrared absorbing agent, a radical polymerization initiator and a polymerizable compound is described in Patent Document 3.
In the lithographic printing plate precursor of the on-press development type, it is extremely difficult to achieve a good balance between on-press development property and printing durability and therefore, a further improvement has been desired.
In order to achieve the good balance between on-press development property and printing durability, it is known to provide an intermediate layer containing a polymer compound including a repeating unit (A) having a support-adsorbing group and a repeating unit (B) having an alkylene oxy unit (see Patent Document 4). However, the on-press development property does not yet reach a sufficient level according to the technique.
It is also known that the water permeability of layer surface increases to improve the on-press development property by adding a specific compound having a polyoxyethylene group or polyoxypropylene group to an intermediate layer provided between a support and an image-recording layer or the image-recording layer (see Patent Document 5). However, it is found that in case of preserving the lithographic printing plate precursor, particularly when the lithographic printing plate precursor has an overcoat layer or when the lithographic printing plate precursors are preserved with an interleaf inserted therebetween, the specific compound migrates into the overcoat layer or interleaf to decrease the amount of specific compound in the intermediate layer or the image-recording layer, thereby severely deteriorating the on-press development property after the preservation.